1. Field of the Invention
This invention relates to x-ray imaging systems. In a primary application the invention relates to the processing of images derived from a plurality of x-ray measurements.
2. Description of Prior Art
In many imaging application areas, images are constructed as a weighted sum of a plurality of measurements. A prime example are the recent new developments in x-ray imaging. Here measurements are made at different energies and/or at different times. These measurements are then combined to provide selective images representing specific materials.
One example is that of temporal subtraction techniques using digital radiography as described in the publication by C. A. Mistretta and A. B. Crummy, "Diagnosis of Cardiovascular Disease by Digital Subtraction Angiography," in Science, Vol. 214, pp. 761-65, 1981. Here measurements taken before and after the administration of iodine into blood vessels are subtracted to provide an image of vessels alone. Another example is that of energy-selective radiography as described in the publication by L. A. Lehmann, et al., "Generalized Image Combinations in Dual KVP Digital Radiography," in Medical Physics, Vol. 8, pp. 659-67, 1981. Here measurements made at different energies are combined to enhance or eliminate specific materials. A third example is energy-selective computerized tomography as described in the publication by R. E. Alvarez and A. Macovski, "Energy-Selective Reconstructions in X-ray Computerized Tomography," in Physics in Medicine & Biology, Vol. 21, pp. 733-744, 1976. Here sets of measurements are made at two energy spectra and distortion-free reconstructions are made which delineate the average atomic number and density of each region. A fourth example is hybrid subtraction described in U.S. application Ser. No. 6-260,694, filed May 5, 1981 and in the publication by W. R. Brody, "Hybrid Subtraction for Improved Intravenous Arteriography," in Radiology, Vol. 141, pp. 828-831, 1981. Here dual energy measurements are made before and after the administration of iodine. Each dual energy pair is used to eliminate soft tissue. In this way the resultant subtracted vessel images are immune to soft tissue motion.
In each case, where a number of measurements are combined to select specific material properties, the resultant SNR (signal-to-noise ratio) is reduced as compared to the nonselective image. Therefore, these various processing techniques which improve the visualization of disease processes by selecting specific materials have the disadvantage of a reduction in SNR. This reduced SNR can interfere with the ability to visualize regions of disease.
The techniques employed in the present invention are generally related to those employed in color television systems for bandwidth reduction. In color television the high frequency components for all three color channels are derived from the luminance signal. Systems for adding the correct amount of high frequency component to each color channel are described in U.S. Pat. No. 2,833,851 issued to Loughlin and U.S. Pat. No. 4,181,917 issued to Richman. These patents do not, however, address the problem of excessive noise in a selective image derived from a plurality of measurements, such as occurs in many x-ray imaging systems.